1. Field of the Invention
The present invention relates to soldering fluxes and their use in processes for soldering members, particularly metallic members together. The flux compositions of the invention are particularly useful in the field of microelectronics, such as in the preparation of integrated circuit modules and integrated circuit boards.
2. Description of the Prior Art
A common task in the manufacture of microelectronic components involves the manufacture of single chip or multi-chip modules having input/output pins which are inserted into a substrate. The input/output pins provide the needed electrical connections to the integrated circuit chip or chips which are subsequently connected to the substrate or carrier. In other presently known manufacturing processes, a chip is soldered directly to a printed circuit board. With either process, solder flux compositions have typically been applied to the pins in order to connect the component to the selected substrate, for instance, the printed circuit board. Flux compositions are employed to remove oxides from the pins and to prevent the pins from oxidizing when subjected to elevated temperatures for soldering, thereby serving to maintain the solderability of the pins. Once the solder is applied, any flux composition or residue remaining on the pins and substrates must be removed to provide as clean a substrate as possible In the past, this has meant that an extra step of flux removal was necessary in the manufacturing process.
The soldering operation, in general, and flux removal, in particular, is increasingly difficult when applied to microelectronics. The pieces to be joined are extremely small, making cleaning, tinning, post-cleaning and inspection difficult. In some cases, to avoid over heating, only the lead portion of the parts to be joined can be heated during the soldering operation. Cleaning and post-cleaning are difficult due to the small size of the component, their large numbers, and the potential damage to the electronics by the cleaning solutions used, if any. Another problem source results from the fact that many of the known soldering fluxes are corrosive. In the environment of microelectronics, corrosion from any residual flux can ruin an extremely costly device.
Many of the organic water soluble fluxes presently available contain corrosive materials such as halides. A flux composition which contains free halogen can result in conversion to hydroacids and corresponding halide ions by hydrolysis at the soldering temperature. Hydroacids can further react with organic materials present in the flux to free halide ions. Accordingly, if the flux residue is not entirely removed, it will lead to corrosion of the parts soldered.
Because of these problems, so-called "non-activated" rosin fluxes have been used in the past in the microelectronic environment in conjunction with a reducing atmosphere in the soldering equipment. This has not generally provided an acceptable solution, however, since the pure rosin alone is limited in oxide removal capability, can require rework to produce an acceptable product, and often a special soldering atmosphere and/or equipment is required.
To improve the pure rosin flux oxide removal capability, a number of "activated" or "mildly activated" rosin fluxes have been developed. These products have several shortcomings, including the necessity of a cleaning step to ensure the removal of corrosive agents left behind after the soldering operation. For instance, it was often necessary to employ specialized water-based detergents followed by a hot water rinse. These cleaning steps were extremely difficult during the assembly of chips to integrated circuit board where the low stand-off height of the chip to the substrate (typically 0.001 inches-0.004 inches) made it extremely difficult to clean underneath the chip with an aqueous or non-solvent process.
Other of the commercially available low residue fluxes which we have tested have proved to be too thin, running out from under the chip during the manufacturing operation and failing to hold it in place.
The present invention has as its object to provide a soldering flux which thermally dissipates during solder reflow so that no visible residue is left on the printed circuit substrate which would be visible with conventional inspection techniques such as light microscopy, or visual inspection.
Another object of the invention is to provide a "no cleaning" necessary flux which results in manufacturing cost savings and which is environmentally superior, requiring no chemical reclamation or waste treatment
Another object of the invention is to provide a "soldering" flux having a novel flux base component which provides superior wetting properties and which provides a flux which remains tacky after application, holding a precisely aligned chip in proper position for up to 8 hours after placement and through reflow.
Another object is to provide a thermally dissipated soldering flux having the required activity to remove oxides present on the substrate without a reducing atmosphere to thereby promote adequate bonding.
Additional objects, features and advantages will be apparent in the following written description.